The structure of a conventional DPMZM 10 is shown in FIG. 1. As seen therein, the DPMZM 10 comprises an optical input 12 for inputting an optical carrier signal, and an optical output 14 for outputting a QAM-modulated optical signal. Downstream of the optical input 12, the DPMZM 10 branches into a first and a second arm 16, 18, respectively, that are rejoined at the optical output 14, thereby forming what is referred to as an “outer MZM” in the present disclosure.
Within each of the first and second arms 16, 18 of the outer MZM, respective first and second “inner” MZMs 20, 22 are provided. The first inner MZM 20 comprises electrodes 24 for applying a first driving voltage VI for generating an in-phase component E1 of the optical signal to be transmitted. In other words, the first driving voltage VI is intended for modulating the part of the carrier signal propagating along first arm 16 of the outer MZM according to the I-component of a base-band signal, and said driving voltage VI is hence generally an AC-signal. In addition to AC-driving, a bias voltage is applied to the first inner MZM 20. While in practical implementations AC-driving and bias would typically be applied using different electrodes, for simplicity a single set of electrodes 24 is shown in FIG. 1.
Likewise, a pair of electrodes 26 is associated with the second inner MZM 22, for applying a second driving voltage VQ for generating a quadrature component EQ of the optical signal and for also applying a suitable bias voltage. Finally, a set of electrodes 28 is provided in the second arm 18 of the outer MZM in order to introduce a desired phase shift of 90° between the in-phase and quadrature components E1 and EQ of the modulated signal before these I- and Q-modulated signals are combined at the optical output 14.
The amplitudes E1 and E2 of the electrical fields of the portions of the carrier entering the first and second arms 16, 18 of the outer MZM can be modulated in response to the driving voltages VI, VQ to give the in-phase and quadrature components EI, EQ as follows:
                                          E            I                    =                                    sin              ⁡                              (                                                                            π                      2                                        ·                                                                  V                        I                                                                    V                        π                                                                              +                                      φ                    1                                                  )                                      ⁢                          E              1                                      ⁢                                  ⁢                                            E              Q                        =                                          sin                ⁡                                  (                                                                                    π                        2                                            ·                                                                        V                          Q                                                                          V                          π                                                                                      +                                          φ                      2                                                        )                                            ⁢                              E                2                                              ,                                    (        1        )            assuming that the DPMZM device is composed of ideal inner and outer MZMs. As seen herein, the in-phase and quadrature components EI, EQ depend non-linearly from the corresponding driving voltages VI, VQ. Vπ is a device dependent constant and φ1 and φ2 are constant phases which can be adjusted by introducing a suitable bias at the electrodes 24, 26, respectively.
Unfortunately, a DPMZM is far from an ideal device: By its construction principle, it has non-linear input-output characteristics, and due to manufacturing imperfections, it generates cross-talk and amplitude imbalance between in-phase and quadrature components of the output signal. It is seen that the manufacturing imperfections are related to the extinction ratio (ER) of the DPMZM, i.e. the ratio of the maximum and minimum output power at the optical output 14 of the outer MZM over a sweep of the first and second driving voltages VI, VQ. An ideal DPMZM has an infinite ER, but series manufactured MZMs rarely achieve a guaranteed ER greater than 20 dB. With the introduction of new technologies like CMOS Photonics, it can be envisaged that newer and cheaper DPMZMs will become available, but their extinction ratios will likely be even well below 20 dB.
The non-ideal characteristics of the DPMZM impair the quality of the transmit signal and result into a performance penalty depending on the adopted signal constellation. State-of-the-art 100 G (˜100 GB/s) optical systems employ 4-point quadrature amplitude modulation (4 QAM) which tolerates well the imperfections of currently available DPMZMs. However, 200 G and 400 G systems will likely rely on 16 QAM that is very sensitive to DPMZM limitations. Future systems may also employ bigger QAM constellations or orthogonal frequency division multiplexing (OFDM) which suffer from even larger penalties. It would therefore seem that for these applications, an increased manufacturing effort is unavoidable in order to achieve the required signal quality.